1) 10 gallons sounds about right. 15 pounds of rock would be fine. The rough guideline is 1-2 Lbs/gal.
2) Mine are roughly the size of a golf ball.
3) I'm not sure on the max size, but they get considerably larger than the blue legs. Mine have never been a problem, but all crabs are opportunistic feeders. Some people have problems with them, some don't. They also have a tendency to eat snails for their shells.
4) I would wait 6-8 weeks after the last sign of ich. A purple firefish would be a great choice. I love the way they look as well.
5) I'm not sure where they're based, but there's INDMAS - Indiana Marine Aquarium Society, and IRC - Indiana Reefers Club.
6) 350 really isn't bad, especially since there's probably not a real high demand for calcium in your tank. That being said, calcium, alkalinity and magnesium are all related. If one is out of whack, the others will be too.
Always happy to help if I can
